Conventional separation processes recover, isolate, and purify products in virtually every manufacturing sphere, covering a broad spectrum of industries including chemicals, pharmaceuticals, petroleum, electronics, automobile, and aerospace. These tend to be energy intensive and expensive, and represent over 40-70 percent of capital and operating costs, and 45 percent of energy costs in the chemical and petroleum industries, even excluding pollution control costs. Recently, industries have gradually shifted their focus from pollution control or treatment towards water recovery, reuse and recycling, primarily due to the following reasons: severe water shortages and droughts, depletion of fresh-water resources, stricter environmental regulations, rising treatment costs, and increasing spatial constraints. Membrane technologies offer a more viable and energy-efficient alternative to conventional separations with substantial economic and environmental benefits. Processes such as microfiltration, ultrafiltration, nanofiltration and reverse osmosis have attracted significant attention owing to their wide applications in industrial, commercial, and environmental operations.
Despite recent advancements, membrane technologies must overcome several technological challenges before wide implementation, and these include membrane fouling, permeate flux decline, poor separation (rejection) characteristics, and low durability. These challenges necessitate innovations and developments in producing membranes that manifest greater resistance to fouling, yield increased permeate flux, and exhibit better selectivity.
Semipermeable reverse osmosis (RO) membrane mediated processes are among the most effective methods to achieve the energy-efficient removal of salts (Na and monovalent and divalent ions) and other aqueous contaminants. Thin-film composite polyamide (PA) membranes dominate the current market due to their good salt rejection and wide pH tolerance. However, these membranes tend to have low water permeabilities that arise from their rigid cross-linked structure. Such membranes also suffer from low durability, sensitivity to temperature, lack of resistance to microbial attack (biodegradation) and lack of resistance to different classes of fouling. In addition, due to the high pressures required for sustainable and enhanced water flux, energy requirements are high.
The improvement of RO membranes through incorporation of nano sized objects such as carbon nanotubes and zeolites into the membrane has been carried out, but the results have been marginal (Li L et al., J. Membr. Sci., 2004, 243, 401-404; Li L et al., Desalination, 2008, 228, 217-225; Fornasiero F et al., Proc. Natl Acad. Sci. USA, 2008, 105, 17250-17255; Holt J K et al., Science, 2006, 312, 1034-1037).
Accordingly, there is a need for improved RO membranes structures and methods for making such structures.